1. Field of the Invention
The present invention relates generally to an output circuit, and more particularly, to an output circuit, a related detecting circuit, and a video output circuit that can detect whether a load is connected to a connecting port corresponding to the output circuit.
2. Description of the Prior Art
Typically, multimedia playback devices, such as set-up boxes, output video and audio signals to display devices through connecting ports and transmission cables. Since there are a variety of different video signaling standards used by the display devices, a multimedia playback device usually has several connecting ports complying with different video signaling standards, such as, composite video, S-Video, component video, digital visual interface (DVI) and high definition multimedia interface (HDMI) standards. However, in actual use, not every connecting port is not connected to a display device, but output circuits or signal processing circuits corresponding to the “un-connected” connecting port still keeps operating since it is not informed of connection status (i.e., whether the display device is connected to the connecting port). As such, the output circuit or signal processing circuit operates to consume power even if it does not need to outputs signals for the display device. This leads to unnecessary power consumption. To reduce the unnecessary power consumption, there exist several detection techniques for detecting the connection status of the load. These detection techniques further allow the multimedia playback device either to turn off the output circuit/the signal processing circuit or to make the output circuit/the signal processing circuit enter a low power mode while the corresponding connecting port is detected to be un-connected.
A common detection method is to dispose a mechanically-driven switch inside the connecting port. The mechanically-driven switch can generate a current signal or a voltage signal when a connector of a transmission cable is plugged into the connecting port. This is achieved by a contact spring of the mechanically-driven switch trigging a signal generation circuit to be shorted or opened, thereby generated a detection signal. The detection signal is sent back to the output circuit/the signal processing circuit and accordingly controls the operating of the output circuit/the signal processing circuit. The output circuit/the signal processing circuit can be turned off as long as it does not receive the detection signal generated by the mechanically-driven switch. On the other hand, when the transmission cable is plugged into the connecting port, the output circuit/the signal processing circuit will be turned on. However, such mechanically-driven switch and related signal generation circuit increases the implementation cost. Also, this detection method requires the output circuit/the signal processing circuit to provide additional signal pin to receive the detection signal generated by the mechanically-driven switch. Therefore, the layout of the output circuit and the signal process circuit become more complicated, and what is even worse, the mechanically-driven switch may wear down or break down because of repeatedly uses.
FIG. 1 and FIG. 2 further illustrate a conventional detection method, and problems thereof. Please refer to FIG. 1. An output stage of an output circuit 10 could generate an output current I_OUT to an output terminal E_OUT, which is provided to a load 50. A matching resistor R1 is in connection to the output terminal E_OUT. A resistance of the matching resistor R1 is matched to a resistance of an equivalent resistor R2 of the load 50. In such detection method, a comparator compares a voltage V_OUT at the output terminal E_OUT with a threshold voltage value VTH. Accuracy of this detection method is affected by how the load 50 is coupled to the output circuit 10. Part (a) of FIG. 2 illustrates changes in the voltage V_OUT in a case of DC coupling (i.e., the coupling capacitor does not exist), where the output stage of the output circuit 10 is assumed to output a fixed current I_OUT. When the load 50 is connected to the output circuit 10, the matching resistor R1 that is in-series connection to the output terminal E_OUT will turn out to be in parallel connection to an equivalent resistor R2 of the load. This causes a lower voltage V_OUT (having a voltage value Vt1) generated at the output terminal E_OUT. On the other hand, if the load 50 is not connected to the output circuit 10, a higher voltage V_OUT (having a voltage value Vt2) thus generated at the output terminal E_OUT. A comparator of the output circuit 10 could set the threshold voltage VTH to have a value between the voltage values Vt1 and Vt2. Part (b) of FIG. 2 illustrates changes in the voltage V_OUT in a case of AC coupling (i.e., the coupling capacitor exists). In such case, the change in the voltage V_OUT is similar to that is in the case of DC coupling. That is, when the load 50 is not connected to the output circuit 10, the voltage V_OUT rises up. Regardless of the connection of the load 50 with the output circuit 10, the value of the voltage V_OUT is always higher than the value is in DC coupling. When the load 50 is connected to the output circuit 10, the voltage value Vt3 of the voltage V_OUT is higher than the voltage value Vt1 in DC coupling. When the load 50 is not connected to the output circuit 10, the voltage value Vt4 of the voltage V_OUT is higher than the voltage value Vt2 in DC coupling. In other words, the coupling capacitor C introduces a positive offset V_offset to the voltage V_OUT. Hence, in the case of AC coupling, both of voltage values Vt3 and Vt4 are higher than the threshold voltage value VTH. It is improper for the comparator of the output circuit 10 to identify the connection status of the connecting port according to the threshold voltage value VTH that is determined based on the voltage values Vt1 and Vt2. Unless the threshold voltage is designed to vary with the existence of the coupling capacitor C, this is impossible to determine a threshold voltage that is able to satisfy conditions of both AC coupling and DC coupling. However, it is complicated to design a variable threshold voltage and may increase the complexity of the output circuit 10. In view of this, there is a need to improve the conventional detection method.